1. Field of the Invention
The present invention relates to a decoding device having a turbo decoder and an RS decoder serially concatenated, and more particularly, to a device for decoding a signal which has undergone both an RS encoding and a turbo encoding, and a decoding method thereof.
2. Description of the Related Art
Generally, in order to correct an error on a channel, a wireless digital communication system uses a method of adding an error correction code in a transmitting terminal and a method of correcting the error in a receiving terminal. One of the coding methods used for the error correction is a turbo code. The turbo code is employed for a channel in need of a high data rate such as CDMA2000 used in the USA, and W-CDMA used in Europe.
FIG. 1 is a block diagram of a conventional decoding device for decoding a received turbo code.
A signal received through a channel passes an input buffer 10 and is input into a turbo decoder 20. The turbo decoder 20 decodes a turbo code by an iterative decoding method and the decoded signal is transmitted to an output buffer 70.
The turbo code varies in the error correction capacity according to an iteration number of the iterative decoding operation. As the iteration number gets greater, the possibility of error correction increases. However, if the iteration number is too great, a decoding time becomes long and power consumption for decoding increases. Therefore, a controller 40 ceases the iterative decoding once the error correction is performed beyond a particular level.
Two conventional criteria determining methods of ceasing the iterative decoding are described below.
The first method is to predetermine an iteration number and cease the iterative decoding when the iteration number reaches the predetermined iteration number. However, this method may increase the decoding time and power consumption as the unnecessary iterative decoding may be performed even though an error is sufficiently corrected. Additionally, this method has a problem that the desired error correction performance may not be achieved as the iterative decoding can be ceased although the error correction is not completed.
The second method is to have a separate LLR (Log Likelihood Ratio) calculator or CRC (Cyclic Redundancy Check) generator 30 as shown in FIG. 1. In other words, it is a method that, by performing the CRC on the signal decoded during the iterative turbo decoding or producing the LLR of the decoding results, the decoding is ceased when it is determined that the error correction is completed according to the CRC result or when the minimum value of the absolute values of the LLR is higher than a predetermined threshold value.
However, in the method using the CRC, the turbo code should be encoded again according to the CRC method, which may cause a data rate loss and the CRC result may be incorrect.
In addition, the method using the LLR has a difficulty in determining a proper threshold value for ceasing the iteration, and errors may still occur even though the LLR conditions are met.
The iteration control of the turbo decoder by the method mentioned above can be adopted even if the error correction is done by the turbo decoder alone. Recently, a new error correction method using both a turbo decoder and a Reed Solomon Decoder (RS decoder) is suggested (U.S. Pat. No. 6,298,461) where a decoding result of an RS decoder is used to control the iteration of the turbo decoder to get a better decoding result.